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01 0609
Renee Mattie wrote:
> I saw an old Weems and Plath Dalton E6B at Bacon's and snapped it up.
> Imagine my surprise to see that they are still making these things,
> and offering them for sale at http://www.mypilotstore.com/MyPilotStore/secp/22.
>
> On the slide-rule side, the grommet seems to be off-center.  So if I
> set the dial for 60 miles/hour, I'll apparantly go almost 18.1 miles
> in 18 minutes, though almost exactly 9 miles in 9 minutes.
>
> Have I got a good one or a bad one?

A not so good one, I guess. The centering error on mine is barely
detectable, about the width of a line.

I also have a Jeppesen CR-2 flight computer. It's much more compact than
an E-6B because the effect of wind is calculated by a different
principle. With the side of the device that's pictured at the web site,
you separate wind into headwind and crosswind components. Crosswind and
true airspeed are set on a sine scale (around the outside in the
picture) to obtain wind correction angle. Headwind is mentally
subtracted from true airspeed to obtain groundspeed.

By contrast, on an E-6B you construct the wind triangle and read
wind correction angle and groundspeed directly. The solution takes
more space, but workload is less.

Either computer can solve for drift and set in marine navigation. Just
mentally apply a convenient scaling factor of 10, 20, etc. to the values
on the wind and airspeed scales. I solved some of the old Silicon Sea
problems that way.

For time / speed / distance computations both devices have the same
facilities. As usual with slide rules, the user is responsible for
placing the decimal point, so ship speeds are as easy to handle as
airplane speeds. Or car speeds -- I used my E-6B a few days ago to check
speedometer error.

When compressibility is significant (say, above .4 Mach) the CR
computers are superior at converting calibrated airspeed to true
airspeed. E-6Bs assume incompressible flow, so they're inaccurate at jet
cruise speeds. But airspeed computation is simpler, and the error means
nothing at typical light plane speeds.

Deluxe E-6Bs are made of sheet aluminum, while CRs are plastic and can
warp if left in a hot car.

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